S.C.C.O.12/6/20151 IV. NUTRITION A. Major nutrients are glucose and oxygen in: tears, limbal blood...
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Transcript of S.C.C.O.12/6/20151 IV. NUTRITION A. Major nutrients are glucose and oxygen in: tears, limbal blood...
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IV. NUTRITIONIV. NUTRITION
A. Major nutrients A. Major nutrients are glucose and are glucose and oxygen in: tears, oxygen in: tears, limbal blood vessels, limbal blood vessels, aqueous humoraqueous humor
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IV. NUTRITIONIV. NUTRITION
– 1. Glucose is not permeable to the 1. Glucose is not permeable to the superficial epithelium. Glucose can diffuse superficial epithelium. Glucose can diffuse from limbal vessels, however the rate of from limbal vessels, however the rate of consumption is too great for this to provide consumption is too great for this to provide for any more than just the most peripheral for any more than just the most peripheral cornea.cornea.
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IV. NUTRITIONIV. NUTRITION
B. ExperimentsB. Experiments• 1. ingested fluorescein will diffuse out of limbal blood 1. ingested fluorescein will diffuse out of limbal blood
vessels, but never get to the central cornea.vessels, but never get to the central cornea.
• 2. Deep limbal cuts, intended to destroy the limbal 2. Deep limbal cuts, intended to destroy the limbal blood supply, have no effect on the cornea.blood supply, have no effect on the cornea.
• 3. implantation of impermeable plastic discs into the 3. implantation of impermeable plastic discs into the stroma, will eventually lead to degeneration of the stroma, will eventually lead to degeneration of the epithelium and anterior stroma. CONCLUSION----epithelium and anterior stroma. CONCLUSION----Glucose must come from the aqueous humor.Glucose must come from the aqueous humor.
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IV. NUTRITIONIV. NUTRITION
C. Endothelial glucose permeability is C. Endothelial glucose permeability is 10 times that for diffusion of a molecule 10 times that for diffusion of a molecule its size. Therefore glucose uptake its size. Therefore glucose uptake across the endothelium must be across the endothelium must be facilitated.facilitated.
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IV. NUTRITIONIV. NUTRITION
D. Between 65-85% of glucose used by D. Between 65-85% of glucose used by the corneal epithelium is metabolized to the corneal epithelium is metabolized to lactate even under normal conditions. lactate even under normal conditions. Much more lactate is produced when Much more lactate is produced when the epithelium is made hypoxic, i.e. the epithelium is made hypoxic, i.e. during contact lens wear.during contact lens wear.
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IV. NUTRITIONIV. NUTRITION
– E. Oxygen is very permeable to all cell E. Oxygen is very permeable to all cell layers. Removing oxygen from the tears by layers. Removing oxygen from the tears by using a nitrogen goggle or tight fitting contact using a nitrogen goggle or tight fitting contact lens leads to corneal edema and clouding. lens leads to corneal edema and clouding. When the eye is closed oxygen will diffuse When the eye is closed oxygen will diffuse out of the palpebral capillaries to the tears. out of the palpebral capillaries to the tears. Oxygen flux from the aqueous to the cornea Oxygen flux from the aqueous to the cornea is especially important when the eye is is especially important when the eye is closed. Limbal oxygen supply is negligible.closed. Limbal oxygen supply is negligible.
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V. STEADY-STATE O2 V. STEADY-STATE O2 DISTRIBUTIONDISTRIBUTION A. First, carbon A. First, carbon
dioxide distributiondioxide distribution
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V. STEADY-STATE O2 V. STEADY-STATE O2 DISTRIBUTIONDISTRIBUTION B. The steady-state O2 curves are derived B. The steady-state O2 curves are derived
from a complex, non-linear function. If Q = from a complex, non-linear function. If Q = dj/dx and j = Dk(dp/dx) then:dj/dx and j = Dk(dp/dx) then:– Q = Q = (d/dx) Dk(dP / dx) = (d/dx) Dk(dP / dx) = Dk (d^2P / dx^2)Dk (d^2P / dx^2)
• Q = O2 consumption rateQ = O2 consumption rate• d^2P/dx^2 = concentration gradient for O2 across the d^2P/dx^2 = concentration gradient for O2 across the
tissuetissue• D = diffusion coefficientD = diffusion coefficient• k = solubility coefficientk = solubility coefficient
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V. STEADY-STATE O2 V. STEADY-STATE O2 DISTRIBUTIONDISTRIBUTION Dk = oxygen Dk = oxygen
permeabilitypermeability Dk/L = oxygen Dk/L = oxygen
transmissibility of a transmissibility of a membrane (e.g., membrane (e.g., contact lens)contact lens)
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V. STEADY-STATE O2 V. STEADY-STATE O2 DISTRIBUTIONDISTRIBUTION C. In the open eye at sea level, pO2 = 155 C. In the open eye at sea level, pO2 = 155
mm Hg (132 mm Hg in Denver) at the mm Hg (132 mm Hg in Denver) at the tears and 55 mm Hg in the aqueous tears and 55 mm Hg in the aqueous humor.humor.
• Layer = QLayer = Q ml O2/ ml x sec ml O2/ ml x sec %%
• EpiEpi 26.5 x 10-5 26.5 x 10-5 40 40
• Stroma 2.85 x 10-5Stroma 2.85 x 10-5 39 39• EndoEndo 140 x 10-5 140 x 10-5
2121
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V. STEADY-STATE O2 V. STEADY-STATE O2 DISTRIBUTIONDISTRIBUTION Oxygen Tension = P = (QX^2 / 2Dk) + BX Oxygen Tension = P = (QX^2 / 2Dk) + BX
+ C; Master Equation, where B and C are + C; Master Equation, where B and C are constants of integration and are determined constants of integration and are determined by the boundary conditions.by the boundary conditions.
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V. STEADY-STATE O2 V. STEADY-STATE O2 DISTRIBUTIONDISTRIBUTION 1. Open Eye Boundary Conditions1. Open Eye Boundary Conditions
– at x = 0 (aqueous), P = 55 i.e. C = 55at x = 0 (aqueous), P = 55 i.e. C = 55– at x = L (tears), P = 155at x = L (tears), P = 155– 155 = (QL^2 / 2Dk) + BL + 55, solve for B:155 = (QL^2 / 2Dk) + BL + 55, solve for B:– P = (QX^2/2Dk) + (X/L)[100- (QL^2/2Dk)] + P = (QX^2/2Dk) + (X/L)[100- (QL^2/2Dk)] +
5555• = Oxygen tension= Oxygen tension
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V. STEADY-STATE O2 V. STEADY-STATE O2 DISTRIBUTIONDISTRIBUTION 2. Closed Eye Boundary Conditions2. Closed Eye Boundary Conditions
– at x = 0, P= 55at x = 0, P= 55– at x = L, P = 55at x = L, P = 55
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VI. OXYGEN TENSION UNDER VI. OXYGEN TENSION UNDER A CONTACT LENSA CONTACT LENS
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VI. OXYGEN TENSION VI. OXYGEN TENSION UNDER A CONTACT LENSUNDER A CONTACT LENS A.Mathematical determination: Diffusion A.Mathematical determination: Diffusion
through the lens (Fick's Law)through the lens (Fick's Law)– 1. J = dm / dt, J is the mass of substance 1. J = dm / dt, J is the mass of substance
moving/unit time across a boundary or moving/unit time across a boundary or layer, m is mass (grams), t is time or J = D layer, m is mass (grams), t is time or J = D a (dc / dx), where D is the diffusion a (dc / dx), where D is the diffusion coefficient (cm^2 / sec), a is the area, dc is coefficient (cm^2 / sec), a is the area, dc is the concentration (grams/cc) gradient, x is the concentration (grams/cc) gradient, x is the thickness of the layer.the thickness of the layer.
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VI. OXYGEN TENSION VI. OXYGEN TENSION UNDER A CONTACT LENSUNDER A CONTACT LENS
– 2. j = D (dc / dx), small j is the flux per unit 2. j = D (dc / dx), small j is the flux per unit area.area.
– 3. remember that for gases c = kP (Henry's 3. remember that for gases c = kP (Henry's Law of gas partial pressure)(c=dissolved O2 Law of gas partial pressure)(c=dissolved O2 conc., k = O2 solubility, P = O2 tension in conc., k = O2 solubility, P = O2 tension in solution) therefore, J = dm / dt = Dka (dP / solution) therefore, J = dm / dt = Dka (dP / dx), if L is the thickness of a contact lens dx), if L is the thickness of a contact lens thenthen• j = Dk (dP / dL) and Dk/L represents the oxygen j = Dk (dP / dL) and Dk/L represents the oxygen
transmissibility of the lens.transmissibility of the lens.
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VI. OXYGEN TENSION VI. OXYGEN TENSION UNDER A CONTACT LENSUNDER A CONTACT LENS B. Experimental Determination of B. Experimental Determination of
Oxygen Uptake Oxygen Uptake – BackgroundBackground– 1. We can measure the oxygen tension of 1. We can measure the oxygen tension of
a solution with an oxygen sensitive a solution with an oxygen sensitive electrode. Over the electrode is placed a electrode. Over the electrode is placed a thin plastic membrane that protects the thin plastic membrane that protects the electrode & acts as a reservoir for oxygen.electrode & acts as a reservoir for oxygen.
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VI. OXYGEN TENSION VI. OXYGEN TENSION UNDER A CONTACT LENS UNDER A CONTACT LENS The oxygen in the solution equilibrates The oxygen in the solution equilibrates
with the membrane and then diffuses to with the membrane and then diffuses to the electrode to produce a steady the electrode to produce a steady current, proportional to the oxygen current, proportional to the oxygen tension in the solution.tension in the solution.
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– 2. Good for measuring O2 in large volumes 2. Good for measuring O2 in large volumes where the electrode is not affecting the where the electrode is not affecting the solution pO2. Want to know the O2 tension solution pO2. Want to know the O2 tension under a contact lens. The problem is that under a contact lens. The problem is that the tear volume is similar to the membrane the tear volume is similar to the membrane reservoir volume & the tear pO2 cannot be reservoir volume & the tear pO2 cannot be maintained if the electrode is pressed to maintained if the electrode is pressed to the eye. If an electrode is pressed against the eye. If an electrode is pressed against the eye the oxygen in the membrane will the eye the oxygen in the membrane will diffuse out into the cornea.diffuse out into the cornea.
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– 3. The cornea will consume the O2 and 3. The cornea will consume the O2 and since the electrode is blocking O2 coming since the electrode is blocking O2 coming from the air, the O2 in the membrane will from the air, the O2 in the membrane will be depleted within a minute or so.be depleted within a minute or so.
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– 4. Suppose a contact lens is worn for a time (5 -10 4. Suppose a contact lens is worn for a time (5 -10 min) so that the oxygen tension of the cornea is min) so that the oxygen tension of the cornea is reduced. When the lens is removed and the reduced. When the lens is removed and the electrode is placed on the eye, the oxygen in the electrode is placed on the eye, the oxygen in the membrane may come out faster than if a lens membrane may come out faster than if a lens hadn't been worn since the difference in O2 hadn't been worn since the difference in O2 tension between the membrane and the cornea is tension between the membrane and the cornea is greater. This in fact occurs, and the absolute O2 greater. This in fact occurs, and the absolute O2 flux from the membrane can be calculated.flux from the membrane can be calculated.
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VI. OXYGEN TENSION VI. OXYGEN TENSION UNDER A CONTACT LENSUNDER A CONTACT LENS C. Experimental MethodsC. Experimental Methods
– 1. Equivalent Oxygen Percentage (R. Hill)1. Equivalent Oxygen Percentage (R. Hill)• a. An empirical method for estimating tear pO2 a. An empirical method for estimating tear pO2
in humans wearing contact lenses.in humans wearing contact lenses.• b. Bathe the cornea of a subject with various b. Bathe the cornea of a subject with various
levels of oxygen. Whip off the goggle and levels of oxygen. Whip off the goggle and measure the oxygen uptake rate. Create a measure the oxygen uptake rate. Create a calibration curve showing tear pO2 vs. O2 calibration curve showing tear pO2 vs. O2 uptake.uptake.
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VI. OXYGEN TENSION VI. OXYGEN TENSION UNDER A CONTACT LENSUNDER A CONTACT LENS
– 1. Equivalent 1. Equivalent Oxygen Percentage Oxygen Percentage (R. Hill)(R. Hill)
• Next place a contact Next place a contact lens on the eye, allow lens on the eye, allow the O2 to equilibrate the O2 to equilibrate for about 10 minutes, for about 10 minutes, whip off the lens and whip off the lens and measure the O2 measure the O2 uptake, find the uptake, find the equivalent pO2 on equivalent pO2 on the calibration curve.the calibration curve.
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VI. OXYGEN TENSION UNDER VI. OXYGEN TENSION UNDER A CONTACT LENSA CONTACT LENS 2. O2 Sensitive 2. O2 Sensitive
Phosphorescence Phosphorescence Dye.Dye.
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VI. OXYGEN TENSION UNDER A VI. OXYGEN TENSION UNDER A CONTACT LENSCONTACT LENS
D. Effect of BlinkingD. Effect of Blinking
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VI. OXYGEN TENSION VI. OXYGEN TENSION UNDER A CONTACT LENSUNDER A CONTACT LENS E. HypoxiaE. Hypoxia
– 1. When cells are 1. When cells are made hypoxic, ATP made hypoxic, ATP production from production from mitochondria slows mitochondria slows and glycolysis and glycolysis speeds up to try to speeds up to try to maintain the ATP maintain the ATP supply. This is called supply. This is called the Pasteur Effect. the Pasteur Effect.
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VI. OXYGEN TENSION UNDER VI. OXYGEN TENSION UNDER A CONTACT LENSA CONTACT LENS As a consequence, As a consequence,
lactate and proton lactate and proton production will production will increase and increase and glucose glucose consumption will consumption will increase.increase.
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VI. OXYGEN TENSION VI. OXYGEN TENSION UNDER A CONTACT LENSUNDER A CONTACT LENS E. HypoxiaE. Hypoxia
– 2. factors limiting the rate of glycolysis 2. factors limiting the rate of glycolysis under hypoxic conditions; glucose supply, under hypoxic conditions; glucose supply, diffusion, glycogen storesdiffusion, glycogen stores
– 3. Epithelial cells store glycogen, which will 3. Epithelial cells store glycogen, which will last about 2 hours during severe hypoxia.last about 2 hours during severe hypoxia.
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VI. OXYGEN TENSION VI. OXYGEN TENSION UNDER A CONTACT LENSUNDER A CONTACT LENS F. Corneal EdemaF. Corneal Edema
– 1. Epithelium1. Epithelium• a. Haze seen in area covered by contact lens.a. Haze seen in area covered by contact lens.• b. Haze is eliminated by removing epithelium or b. Haze is eliminated by removing epithelium or
reduced by rolling a Q-tip across the cornea.reduced by rolling a Q-tip across the cornea.• c. Epithelial thickness does not change, so water c. Epithelial thickness does not change, so water
is not added between the layers of epithelium.is not added between the layers of epithelium.• d. Haze is seen subjectively as colored fringes d. Haze is seen subjectively as colored fringes
around bright lights. This is called Sattler's Veil.around bright lights. This is called Sattler's Veil.
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VI. OXYGEN TENSION VI. OXYGEN TENSION UNDER A CONTACT LENSUNDER A CONTACT LENS
– 1. Epithelium1. Epithelium• e. Haze is due to light scatter at the level of the e. Haze is due to light scatter at the level of the
basal cells. Photographs of basal layer with the basal cells. Photographs of basal layer with the specular microscope reveal a matrix of light specular microscope reveal a matrix of light scattering sites which act as a diffraction scattering sites which act as a diffraction grating. This grating produces a diffraction ring grating. This grating produces a diffraction ring that subtends a half angle of 3 degrees using that subtends a half angle of 3 degrees using red light, 632.8 nm.red light, 632.8 nm.
• f. Thus, increase in water between the basal f. Thus, increase in water between the basal cells.cells.
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VI. OXYGEN TENSION VI. OXYGEN TENSION UNDER A CONTACT LENSUNDER A CONTACT LENS
– 2. Stromal edema2. Stromal edema• a. Cornea will swell, i.e. increase in thickness, a. Cornea will swell, i.e. increase in thickness,
during hypoxia or contact lens wear. during hypoxia or contact lens wear. Possibilities:Possibilities:
– i. Hypoxia reduces epi- or endothelial barrier i. Hypoxia reduces epi- or endothelial barrier function.function.
– ii. Endothelial pump is slowed directly by hypoxia.ii. Endothelial pump is slowed directly by hypoxia.– iii. Corneal pH drop slows endothelial pump.iii. Corneal pH drop slows endothelial pump.– iv. Lactate accumulation causes osmotic swelling.iv. Lactate accumulation causes osmotic swelling.
• b. Cornea will swell about 4% during sleep, i.e. b. Cornea will swell about 4% during sleep, i.e. eye closure.eye closure.
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VI. OXYGEN TENSION VI. OXYGEN TENSION UNDER A CONTACT LENSUNDER A CONTACT LENS
– 2. Stromal edema2. Stromal edema• c. Acute hypoxia does not compromise the c. Acute hypoxia does not compromise the
epithelial barrier, however chronic severe epithelial barrier, however chronic severe hypoxia will. In addition, chronic mild hypoxia hypoxia will. In addition, chronic mild hypoxia can slow the epithelial mitotic and healing rates. can slow the epithelial mitotic and healing rates. All of these problems increase the chance for All of these problems increase the chance for corneal infection. The effect of reduced pH on corneal infection. The effect of reduced pH on these processes is not known, but it could be a these processes is not known, but it could be a contributing factor.contributing factor.
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VI. OXYGEN TENSION VI. OXYGEN TENSION UNDER A CONTACT LENSUNDER A CONTACT LENS
– 3. Endothelial Changes from contact lens wear3. Endothelial Changes from contact lens wear• a. Blebs are transitory dark spots appearing in the a. Blebs are transitory dark spots appearing in the
specular reflection of the endothelium. Occur about specular reflection of the endothelium. Occur about 30 min. after insertion of a contact lens and last about 30 min. after insertion of a contact lens and last about 30-60 min. Most likely due to pH changes.30-60 min. Most likely due to pH changes.
• b. Polymegathism, increased variation in cell size b. Polymegathism, increased variation in cell size seen in long-term lens wearers:seen in long-term lens wearers:
– coefficient of variation = S.D. of cell size / mean cell sizecoefficient of variation = S.D. of cell size / mean cell size
• c. Pleomorphism – increased variation in cell shapec. Pleomorphism – increased variation in cell shape
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VI. OXYGEN TENSION UNDER VI. OXYGEN TENSION UNDER A CONTACT LENSA CONTACT LENS
• d. Reduced function due to hypoxic dosed. Reduced function due to hypoxic dose– Corneal deswellingCorneal deswelling
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VI. OXYGEN TENSION VI. OXYGEN TENSION UNDER A CONTACT LENSUNDER A CONTACT LENS
– 4. Critical O2 4. Critical O2 TensionTension
• a. Oxygen levels a. Oxygen levels below this result in below this result in adverse tissue adverse tissue changes.changes.
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VII. TISSUE MECHANICSVII. TISSUE MECHANICS
A. Cornea has a high tensile strength and A. Cornea has a high tensile strength and can resist rupture up to 30 Atm.can resist rupture up to 30 Atm.
B. Cornea cannot support lateral shearing B. Cornea cannot support lateral shearing forces. Lamellae tend to slide past each forces. Lamellae tend to slide past each other & its relatively easy to slide a probe other & its relatively easy to slide a probe into the stroma between lamellae.into the stroma between lamellae.
C. Sclera is weaker, especially at EOM C. Sclera is weaker, especially at EOM insertions.insertions.
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VII. TISSUE MECHANICSVII. TISSUE MECHANICS
D. The total force due to the wall D. The total force due to the wall tension is the product of the tension, T tension is the product of the tension, T (Force / cm), and the circumference, 2 (Force / cm), and the circumference, 2 Pi(r) of the hemisphere, or 2 Pi(r)T. At Pi(r) of the hemisphere, or 2 Pi(r)T. At equilibrium, the force to the left must equilibrium, the force to the left must equal the force to the right. The force equal the force to the right. The force vectors due to the IOP act vectors due to the IOP act perpendicular to the surface of the wall.perpendicular to the surface of the wall.
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VII. TISSUE MECHANICSVII. TISSUE MECHANICS
D. Each force vector can be D. Each force vector can be represented by a vertical & horizontal represented by a vertical & horizontal component. All the vertical components component. All the vertical components cancel, those to the right add up to the cancel, those to the right add up to the IOP times the projected area, Pi(r)^2. IOP times the projected area, Pi(r)^2. Therefore at equilibrium:Therefore at equilibrium:
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VII. TISSUE MECHANICSVII. TISSUE MECHANICS
– Force on the wall = 2 Pi(r)T = (IOP) Pi(r)^2 Force on the wall = 2 Pi(r)T = (IOP) Pi(r)^2 and and
– T = (IOP) (Pi(r)^2 / 2 Pi(r)) = (IOP) r / 2T = (IOP) (Pi(r)^2 / 2 Pi(r)) = (IOP) r / 2• This result predicts that the tension on the This result predicts that the tension on the
cornea and sclera for the human eye are cornea and sclera for the human eye are different.different.
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VII. TISSUE MECHANICSVII. TISSUE MECHANICS
E. The corneo-scleral junction is stable, E. The corneo-scleral junction is stable, so there must be a compensating so there must be a compensating tension at the limbus. This is provided tension at the limbus. This is provided by the circular fibers.by the circular fibers.
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VII. TISSUE MECHANICSVII. TISSUE MECHANICS
F. Radial Keratotomy makes cuts 90-F. Radial Keratotomy makes cuts 90-95% through the cornea. This weakens 95% through the cornea. This weakens the cornea allowing the IOP to push out the cornea allowing the IOP to push out the weakened area.the weakened area.
G. PRK removes a portion of the G. PRK removes a portion of the stroma.stroma.
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VIII. DISEASEVIII. DISEASE
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VIII. DISEASEVIII. DISEASE
A. Epithelial wound healingA. Epithelial wound healing– 1. The goal is to re-establish the anterior surface 1. The goal is to re-establish the anterior surface
seal, i.e. tight junctions as quickly as possible.seal, i.e. tight junctions as quickly as possible.– 2. Following a wound, the superficial cells at the 2. Following a wound, the superficial cells at the
wound margin will slough off, loss of wound margin will slough off, loss of hemidesmosomes occurs, latent phase approx. hemidesmosomes occurs, latent phase approx. 2 hours.2 hours.
– 3. The basal cells at the margin begin to flatten 3. The basal cells at the margin begin to flatten and spread into the wound.and spread into the wound.
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VIII. DISEASEVIII. DISEASE
– 4. While spreading continues, mitotic activity is 4. While spreading continues, mitotic activity is enhanced in an area surrounding the wound, enhanced in an area surrounding the wound, but removed from the edge. (Mitotic activity is but removed from the edge. (Mitotic activity is slowed by sensory denervation or contact lens slowed by sensory denervation or contact lens wear).wear).
– 5. Cell attachment achieved via transitory 5. Cell attachment achieved via transitory fibronectin & laminin.fibronectin & laminin.
– 6. Once contact inhibition is achieved 6. Once contact inhibition is achieved hemidesmosomes are established.hemidesmosomes are established.
– 7. Therapy is Pressure patching7. Therapy is Pressure patching
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VIII. DISEASEVIII. DISEASE
B. Recurrent ErosionB. Recurrent Erosion– 1. Damage to underlying basement membrane 1. Damage to underlying basement membrane
prevents good adhesion of epithelium even prevents good adhesion of epithelium even after wound is closed. May take several weeks after wound is closed. May take several weeks to months for BM to be repaired. Therapies:to months for BM to be repaired. Therapies:• a. hyperosmotic drops (high MW)a. hyperosmotic drops (high MW)• b. fibronectin dropsb. fibronectin drops• c. EGFc. EGF• d. aprotinin, a plasmin blockerd. aprotinin, a plasmin blocker
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C. Endothelial Cell DamageC. Endothelial Cell Damage
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C. Endothelial Cell DamageC. Endothelial Cell Damage